Automata, Computability, & Complexity
by Elaine Rich
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The Big Picture:
A Language Hierarchy
Chapter 3
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Generator vs. Recognizer
Reminder…
Given a problem, we can develop a
machine (automaton) that
• Generates solutions, or
• Recognizes a solution
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Generator vs. Recognizer
Example
Given 2 integers A & B, determine the sum.
• Generator: Write a program to accept A &
B as input then compute the sum A+B
• Recognizer: Write a program to accept A &
B & C as input then determine if A+B = C
We usually write Generators! But when
would an Recognizer be an appropriate
solution?
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Decision Problems
A decision problem is simply a problem for which the
answer is yes or no (True or False).
A decision procedure answers a decision problem.
Example
• Given an integer n, does n have a pair of consecutive
integers as factors?
The language recognition problem: Given a
language L and a string w, is w in L?
Our focus
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Encoding
Not the same as “coding”, i.e. writing a computer program!!
“Everything is a string”
Do you believe that statement?
What about computer memory?
“Most problems in computing can be
converted to a string”
How? By a correct encoding.
Thus, we can develop a decision solution.
Problems that don’t look like decision problems can
be recast into new problems that are decision.
E.G. A+B=C
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Notation for Encoding into Strings
Almost anything can be encoded as a string.
Let X & Y be some type of “object”.
What is an “object”?
<X> is the string encoding of X.
<X, Y> is the string encoding of pair X, Y.
If we can define a problem as a language (of
strings), we can develop a recognizer. It
becomes a decision problem.
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Example of Encoding
Pattern matching on the web
Problem: Given a search string w and a web
document d, do they match? In other words,
should a search engine, on input w, consider
returning d?
The language to be decided:
{<w, d> : d is a candidate match for the query w}
Recognizer vs. Generator?
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Example of Encoding
Does a program always halt?
Problem: Given a program p, written in some
programming language, is p guaranteed to halt on
all inputs?
• The language to be decided:
HPALL = {p : p halts on all inputs}
Classic problem: The Halting Problem. More later!
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Example of Encoding
Testing for prime numbers
Problem: Given a nonnegative integer n, is it
prime?
•The language
PRIMES = {w : w is the binary encoding of
a prime number}.
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Example of Encoding
• Problem: Given an undirected graph G, is it connected?
• Instance of the problem:
1
2
4
3
5
• Encoding of the problem: Let V be a set of binary numbers, one for
each vertex in G. Then we construct G as follows:
• Write |V| as a binary number,
• Write a list of edges, each represented by pair of num. corresponding to
vertices it connects (first # tells num. of vertices)
• Separate all such binary numbers by “/”.
101/1/10/10/11/1/100/10/101
• The language to be decided: CONNECTED = {w  {0, 1, /}* : w =
n1/n2/…ni, where each ni is a binary string and w encodes a
connected graph, as described above}.
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Turning Problems Into Decision
Problems
Casting multiplication as decision:
• Problem: Given two nonnegative integers, compute the
product.
• Encoding : Transform computing into verification.
• The language:
L = {w of the form:
<integer1>x<integer2>=<integer3>, where:
<integern> is any well formed integer, and
integer3 = integer1  integer2}
12x9=108
12=12
12x8=108
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Turning Problems Into Decision
Problems
Casting sorting as decision:
• Problem: Given a list of integers, sort it.
• Encoding of the problem: Transform the sorting
problem into one of examining a pair of lists.
• The language to be decided:
L = {w1 # w2: n1
(w1 is of the form <int1, int2, … intn>,
w2 is of the form <int1, int2, … intn>, and
w2 contains the same objects as w1 and
w2 is sorted)}
Examples:
1,5,3,9,6#1,3,5,6,9  L
1,5,3,9,6#1,2,3,4,5,6,7  L
Could we
define sorting
as a different
recognition
problem??
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The Traditional Problems & Their
Language Formulations are Equivalent
Equivalent means either problem can be
reduced to (converted to) the other.
Given a machine to solve one, a machine
to solve the other can be built using the
first machine & other functions that can be
built using a machine of equal or lesser
power.
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An Example
Consider the multiplication example:
L = {w : <integer1>x<integer2>=<integer3>, where:
<integern> is a well-formed integer &
integer3 = integer1  integer2}
Given a multiplication machine, we can build the
language recognition machine.
Given the language recognition machine, we can
build a multiplication machine.
This is not saying each machine is efficient!
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One Hierarchy of Languages
D=decidable
SD = Semidecidable
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Chomsky Hierarchy of Languages
Languages from “simplest” to “complex”
Each is a subset of the ones below
• Regular
• Context Free
• Context Sensitive
• Recursively Enumerable
Can be defined by the type of
Machine that will recognize it.
Noam Chomsky
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Regular Languages
A Regular Language is one that can be recognized by a
Finite State Machine.
An FSM to accept a*b*:
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Context Free Language
A Context Free Language is one that can be
recognized by a Push Down Automata.
A PDA to accept AnBn = {anbn : n  0}
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Decidable & Semidecidable
Languages
A Decidable Language is one that is
recognized by a Turing Machine which halts
on all input strings.
A Semidecidable Language is one that is
recognized by a Turing Machine which halts
on all input strings which are in the
language, but may loop infinitely on some
strings which are not in the language
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Turing Machines
R/W head
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Why do we care?
Rule of Least Power: Use the least powerful
machine for expressing a language.
• Efficiency – time & space
• Decidability – other comparisons
• Clarity – descriptive tools
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Languages and Machines
Rule of Least Power: “Use the least powerful
language suitable for expressing information,
constraints or programs on the World Wide Web.”
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Tractability
• A problem is said to be intractable if the
time to solve the problem is exponential
• P: polynomial
• NP: non-deterministic polynomial
• P  NP
• Open Question: Is P = NP?
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Chapter 3 Homework
• Page 34 – Problems 2 & 3
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